Figure 1 Figure 2 Figure 3 Figure 4
Table 1 Table 2 Table 3a Table 3b

 

  

GEOLOGICAL SURVEY OF CANADA

OPEN FILE 4889

  

 

Apatite fission track age, length and kinetic parameter (Cl, Dpar)

data for the Northrock et al. East MacKay I-77 well,

central Mackenzie Valley, Northwest Territories

 

 

  D.R. Issler

Natural Resources Canada, Geological Survey of Canada, 3303-33rd Street NW, Calgary, AB T2L 2A7

 

and

 

A.M. Grist

Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 4J1

 

2005

                                                                                   

 

Abstract

                                                                                               

            Apatite fission track (FT) age and length measurements for a sandstone cuttings sample from the Devonian Imperial Formation of the Northrock et al. East MacKay I-77 well (64o 46' 41.61" N latitude; 125o 43' 10.28" W longitude) are presented herein. The well is located in the central Mackenzie Valley, Northwest Territories, south of Tulita and east of the MacKay range. Also included are compositional (electron microprobe elemental data) and etch figure size (Dpar) data for apatite age and length grains that are used for defining FT age populations with different thermal annealing behaviour. Two different kinetic populations with pooled FT ages of 90.4±6.1 Ma and 222.2±22.5 Ma are well defined on the basis of Cl content. A Cl value of 0.125 atoms per formula unit (apfu) (0.45 wt%) separates these two FT kinetic populations whereas FT ages show considerable overlap when plotted with respect to the Dpar parameter.

 

Introduction

 

            This report presents supplementary data and plots to accompany the paper by Issler et al. (in press) on a multi-kinetic apatite fission track (FT) thermal history study of the East MacKay I-77 well in the central Mackenzie Valley, Northwest Territories. Data include apatite FT age (Table 1) and length (Table 2) measurements with corresponding kinetic parameter data for a sandstone cuttings sample from the depth interval, 1730-1780 mKB. Also included are elemental data for apatite grains originally collected as wt % (Table 3a) and recalculated in terms of atoms per formula unit (apfu; Table 3b). The I-77 well is situated in the Keele Tectonic Zone (MacLean and Cook, 1999), a region with a history of anomalous subsidence and exhumation throughout the Phanerozoic. The well penetrated Upper Cretaceous foreland strata resting unconformably on Upper Devonian strata of the Imperial Formation. The FT sample is interesting because it consists of two age populations with different thermal annealing properties that permit enhanced resolution of its post-Devonian thermal history. Very little has been published on multi-kinetic apatite FT data; the I-77 data provide a well-documented example.

 

Apatite FT Age Data

 

            Table 1 lists apatite grain age information along with the measured kinetic parameters, Cl content (from Table 3b), and Dpar, the arithmetic mean maximum diameter of FT etch figures parallel to the crystallographic c-axis (Donelick, 1993; Burtner et al., 1994). Data acquisition procedures are described in Issler et al. (in press). Apatite composition is known to affect temperature-dependent FT annealing (e.g. Green et al., 1986; Carlson et al., 1999; Barbarand et al., 2003; Ravenhurst et al., 2003). Generally, increased Cl content causes increased resistance to annealing except at very high concentrations where the trend reverses (Carlson et al., 1999; Gleadow et al., 2002; Kohn et al., 2002). Dpar depends on apatite solubility which in turn is a function of apatite composition; therefore it is a proxy kinetic parameter for FT annealing. Table 1 contains both measured and corrected Dpar values; the correction is meant to compensate for differences in laboratory etching conditions and observer bias so that the data are equivalent to those of Carlson et al. (1999) for use with the Ketcham et al. (1999) annealing model. Measured Dpar values are multiplied by the ratio of average Dpar values for Durango apatite as reported by Carlson et al. (1999) (1.83 mm) and as measured in this study (1.74 mm). The effect is to increase measured Dpar values by approximately 5%.

            OH content has been used as a kinetic parameter for FT annealing by Ketcham et al. (1999) and calculated OH values (from Table 3b) are included in Table 1. The parameter, rmro, is a measure of the relative resistance to annealing among apatite grains (Table 1). It forms the basis for the multi-kinetic FT annealing model which compares the degree of apatite FT annealing relative to that for the most retentive end-member apatite composition. It is defined as the reduced length of the most resistant apatite at the point where the less resistant apatite first becomes totally annealed. rmro values were calculated using the empirical equation of Carlson et al. (1999),


  (1)

which uses apfu elemental values in Table 3b for a Ca10(PO4)6F2 end-member. Others is the sum of the cations Na, Mg, Sr, Y, Ce and La (Table 3b). rmro is also expressed in terms of Cl and Dpar using the equations (Ketcham et al., 1999, 2000),


  (2)

and
  (3)

Calculated rmro values from equation 1 were substituted into equations 2 and 3 to obtain calculated Cl and Dpar values for comparison with measured values (Table 1).

            A plot of apatite FT grain age versus measured Dpar (Figure 1A) indicates that different age populations cannot be resolved using Dpar; ages are grouped according to their similarity and show considerable overlap with respect to Dpar. In contrast, a younger (90.4±6.1 Ma) and older (222.2±22.5Ma) FT age population are resolved when grain ages are plotted with respect to Cl content (Figure 1B). Three anomalous high Cl grains share attributes of both populations and are likely caved from the Cretaceous section. A plot of Cl versus Dpar for age (Figure 2A) and length (Figure 2B) grains shows considerable scatter; the slopes of the linear regression equations (calculated using the reduced major axis method; e.g. Davis, 1986) are very similar to the relationship deduced from equations 2 and 3 except that the lines are offset by approximately 0.2 mm on the Dpar axis. This may indicate that our Dpar correction has not fully compensated for differences between our measurement techniques and those of Carlson et al. (1999).

            A plot of FT grain age versus number of grid counting squares for track density measurement (Quads; Fig. 3A and Table 1) indicates that the older FT grains are associated to 16 or less grid squares whereas the younger grains span from 4 to 80 grid squares. The number of grid squares is a proxy measure for grain size; thus, older FT grains are silt-sized whereas the younger grains vary from silt- to sand-size. Similarly, high Cl content (with the exception of a few outliers; Fig. 3B and Table 1) is associated with silt-sized grains whereas lower Cl values occur over a broader range of grain sizes. These results are in accord with the multi-stage sample processing history; the older silt-sized fraction was recovered when the sample was reprocessed with new plates on the rock crusher (Issler et al., in press). The differences in age, Cl content and grain-size strongly indicates that this sample contains different kinetic populations with different thermal annealing behaviour.

 

Apatite FT Length Data

 

            Table 2 contains all the horizontal, confined (etched FTs parallel and below the polished mineral surface) FT length measurements, their orientation with respect to the crystallographic c-axis, and the associated measured and calculated kinetic parameters (Dpar, Cl, OH, rmro) as described above. Sixty lengths are associated with kinetic population 1 fluorapatite grains (mean length is 10.73±2.14 mm); kinetic population 2 Cl-rich grains have 111 FT length measurements (mean length is 10.41±1.99 mm). Apatite grains for three additional lengths were not probed but do have Dpar data (Table 2). Most of the length measurements come from two separate grain mounts, irradiated with 252Cf, to increase the number of etchant pathways for enhanced revelation of horizontal tracks. Additional FT lengths were obtained from three different age mounts (Issler et al., in press).

            FT annealing is anisotropic (e.g. Green et al., 1986; Galbraith and Laslett, 1988; Galbraith et al., 1990; Donelick, 1991) and therefore horizontal FT lengths can be corrected back to a standard orientation (parallel to crystallographic c-axis) using an appropriate length projection model (Donelick et al., 1999; Ketcham, 2003). A plot of measured FT length (Fig. 4A) and FT lengths projected parallel to the mineral c-axis (Fig. 4B) versus Cl shows little structure in the data, consistent with two kinetic populations that experienced similar degrees of annealing but at different temperatures and times (see modelling results of Issler et al., in press). For modelling purposes, the measured track lengths in Table 2 were corrected by Issler et al. (in press) in a similar manner to the Dpar measurements as described above.

 

Apatite Elemental Data

 

            Table 3a lists the raw elemental wt% data obtained from the JEOL electron microprobe at Dalhousie University. Single spot analyses were done using wavelength-dispersive methods with an accelerating voltage of 15 kV, a beam current of approximately 20.6 nA and a beam width of 5 mm (an initial beam width of 10 mm yielded low elemental totals for the silt-sized apatite grains). With a few exceptions, duplicate analyses for selected grains showed good reproducibility (generally 0.01-0.05 wt% for Cl values <0.6 wt%). Table 3a includes some low elemental totals but these data were still useful for sorting the apatite grains into high Cl and low Cl populations. Despite the wide variation in Cl content for kinetic population 2 (0.45 to 2.25 wt%), an effective Cl concentration of 0.21 apfu (0.71 wt%) could be used to model the data (Issler et al., in press). This range in Cl content is far greater than the uncertainty in analytical precision on single grain measurements.

            Elemental wt% data were converted to apfu (Table 3b) in a spreadsheet assuming a 42 atom total based on the end-member formula, Ca10(PO4)6F2, full occupancy of the halogen site (OH determined by iteration using measured O) and various cation substitutions. We used the following standard procedures: (1) elemental wt% values were converted to atomic proportions by dividing by the atomic weight; (2) apfu values were calculated by dividing the atomic proportions by the sum of atomic proportions and multiplying by 42 atoms; (3) OH was calculated iteratively using the criterion, F+Cl+OH=2; (4) O values were adjusting according to the amount of O incorporated into OH. The quality of analyses was judged using the wt% totals and the total for cations and anions based on the idealized apatite formula. Also listed in Table 3b are the kinetic parameters, rmro, Cl and Dpar, calculated using equations 1 to 3.

 

Acknowledgements

 

            We thank Art Stirrett of Northrock Resources Ltd. for encouraging us to undertake this study and for supplying geological information. We also thank Ray Donelick for granting us permission to use his patented Dpar technique. Kim Dunn is thanked for her technical assistance in the final production of this report. The study was funded by Northrock Resources Ltd. and the Program on Energy Research and Development (PERD) under the Northern Resources Development Program of the Geological Survey of Canada..

 

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